The present invention is a method for measuring and controlling the temperature of a semitransparent layer during its deposition onto a semiconductor wafer. During its deposition this layer will be referred to as a "growing layer."
Accurate control of the temperature of a growing layer is essential to the production of a high quality semiconductor device product. Changes in temperature during the deposition process cause variations in the thickness and crystal structure of the resultant finished layer. As the dimensions of active elements and leads has continued to decrease in modern integrated circuits, the importance of accurately controlling the thickness of the several layers which compose a semiconductor device structure has steadily increased. Techniques in which a wafer is moved sequentially through a series of reaction chambers, each designed to treat a single wafer at a time ("single wafer processing techniques") have been developed to obtain smaller device geometries. Unfortunately, accurate temperature control in a single wafer reaction chamber, which is designed to quickly reach a very high temperature, is difficult to achieve for reasons explained below.
Essential to the control of the growing layer temperature is the accurate measurement of the growing layer temperature. The growing layer can not be physically contacted by a temperature sensor because to do so would affect the heat transfer near the point of contact and corrupt the measurement. Efforts to remotely measure the surface temperature have centered on the measurement of infrared light (IR).
For all solids the amount of light that is emitted (radiated) from a surface is related to the temperature of the surface by a parameter known as emissivity. For an opaque solid emissivity does not change with thickness and is a constant having a value of up to 1.0 depending on the type of material. For example, a bare silicon wafer surface has an emissivity of 0.66 to 0.68.
As a growing layer of semitransparent material is deposited on a silicon wafer, the emissivity of the surface changes with growing layer thickness due to optical interferences from reflections at the wafer surface, underlying layer surfaces and the growing layer surfaces. At a single wavelength used to measure the temperature, the emissivity can vary by as much as 0.2-0.9 leading to measurement errors of several tens of degrees. The variation of the emissivity also affects the heat losses radiated from the front surface of the wafer. If the temperature is not measured and used to adjust the heat supplied to the wafer to compensate for the changes in the thermal radiation from the front surface of the wafer, the wafer temperature can vary by as much as 20 degrees Celsius. In these types of depositions, and in most semiconductor processing, a temperature control within a few degrees Celsius is required.
Three basic approaches have been taken to overcome the problems posed in trying to determine temperature by measuring IR light intensity in a test environment of changing emissivity. The original approach utilized a broad band of light between 600 and 1000 nm to attempt to average the variations of surface emissivity at each wavelength caused by variation in the degree of constructive and destructive interference in the growing layer.
The second approach attempts to measure emissivity by shining an active light source at the same wavelength used to measure temperature at the wafer surface and measuring the reflected light. Since the emissivity plus the reflectivity of a surface must equal one, the emissivity of the surface is deduced from the value of the reflectivity. When the emissivity is known, the temperature can be easily obtained from the amount of light emitted from the surface in the absence of the incident light. The difficulty with this technique is that the measured reflectivity is a strong function of the roughness of the surface and patterned process wafer surfaces become quite rough as sequential layers are deposited. Due to this surface roughness, the incident light rays are reflected in many directions and the light reflected into the measuring optics is not predictably related to the emissivity of the surface by a simple equation. Efforts to date have had limited success.
The third approach uses an active light source with wavelengths which are different than the wavelength used to measure the temperature. In this case, many wavelengths are used and analytical models of interferences in multiple layer structures are used to compute the thickness of the growing layer. From this thickness, the emissivity of the surface at the measured wavelength is calculated and used in the temperature measurement. The errors in this approach arise from the accuracy of the analytical models, particularly for the complicated structures found on practical wafers where the device patterns cover the entire wafer surface. Even in the absence of the patterned surfaces, lack of information on the optical properties of the growing layer and the layers underneath the growing layer causes these analytically based approaches to have limited application. In order to model the optical events near the wafer surface accurately, the investigator must know the index of refraction and optical absorption of each layer at the deposition temperature and measured wavelength. In general, this information is not available.
What is, therefore, needed but not yet available is a method for accurately controlling the temperature of a layer of semitransparent material that is being deposited on a semiconductor wafer.